1. Field of the Invention
The invention relates to a method and apparatus for manufacturing metallic parts, more particularly to a method and apparatus for manufacturing metallic parts by a process involving injection of a melted metal into a mold, including die casting methods.
2. Description of the Related Art
One conventional method used to produce molded metallic parts from melted metal is by die casting. Die casting methods use liquid metal during casting and, as a consequence, molded metallic parts produced from this method can have low densities. Molded metallic parts having low densities are not generally desirable because of their reduced mechanical strength, higher porosity, and larger micro shrinkage. It is thus difficult to accurately dimension conventional molded metallic parts and, once dimensioned, to maintain their shapes. Moreover, molded metallic parts produced from conventional die casting have difficulty in reducing the resilient stresses developed therein.
Thixotropic methods for producing molded metallic parts generally improve upon the die casting method by injection molding a metal from its thixotropic (semi-solid) state rather than from its liquid state. The result is a molded metallic part which has a higher density than one produced from the die casting method. Thixotropic methods are disclosed in U.S. Pat. Nos. 3,902,544 and 3,936,298, both of which are incorporated by reference herein.
Methods and apparatuses for manufacturing molded metallic parts from melted metal in its thixotropic state are also disclosed in U.S. Pat. No. 5,501,226 and Japanese patent publications 5-285626 and 5-285627, which are incorporated by reference herein. Methods of converting a metal into a thixotropic state by controlled heating and shearing in an extruder are disclosed in U.S. Pat. Nos. 5,501,226, 4,694,881 and 4,694,882. The systems disclosed in these patent documents are essentially in-line systems, in which the conversion of the metal alloy into a thixotropic state is assisted by an extruder and the pressurizing of the same for the purposes of injection molding; all these steps are carried out within a single cylindrical housing. It is difficult to accurately control all of the process parameters within a single cylindrical housing, especially temperature, shot volume, pressure, time, etc., and as a result, molded metallic parts of inconsistent characteristics are produced.
Moreover, some of these systems require that the metal supplied to the feeder be in pellet form. As a consequence, if a molded metallic part of undesired characteristics is produced by its system, recycling of the defective part is not possible unless the defective part is first recast in pellet form. Furthermore, metal parts made from metal in the thixotropic state which is injected into a mold may have an uneven surface. Such metal parts require further processing before they can be painted.
The present inventor""s co-pending application, Ser. No. 08/873,922, filed on Jun. 12, 1997, which is incorporated by reference herein, describes a different and improved method for producing molded metallic parts from melted metal in a thixotropic state wherein the conversion of melted metal into the thixotropic state takes place in a physically separate location from the location where the metal is injected into the mold and under different conditions.
An improved system for manufacturing molded metallic parts, which is capable of accurately producing molded metallic parts of specified dimensions within a narrow density tolerance that operates with melted metal in a liquid state, is desired. Further, a production process for molded metallic parts that can consistently produce molded metallic parts of desired characteristics and that can easily accommodate recycling of defective parts is desired. Further, an improved production process for molded metallic parts made of lighter metals, like magnesium, is desired.
An object of the invention is to provide a method and apparatus for producing molded metallic parts through injection of melted metal into a mold.
Another object of the invention is to provide an improved injection molding system for producing molded metallic parts that is capable of producing molded metallic parts of accurate dimensions within a narrow density tolerance and operates using melted metal in a liquid state.
Still another object of the invention is to provide an injection molding system for molded metallic parts that is capable of producing metallic parts of desired characteristics in a consistent manner.
Still another object of the invention is to provide an injection molding system that minimizes the amount of gas trapped in liquid metal prior to its injection into the mold.
Still another object of the invention is to provide molded metallic parts having exceptionally smooth surfaces.
Still another object of the invention is to provide molded metallic parts having reduced porosity compared to parts produced by known die casting and thixotropic methods.
Still another object of the invention is to provide molded metallic parts that do not need to be further processed before they are painted.
Still another object of the invention is to provide an injection molding system for producing molded metallic parts that accommodates recycling of defective molded metallic parts easily.
These and other objects are accomplished by an improved injection molding method for producing molded metallic parts comprising the steps of introducing melted metal into a first chamber through a feeder port, allowing at least a portion of the melted metal to flow through said first chamber toward an outlet port, drawing into a second chamber at least a portion of the melted metal through the outlet port under a suction created in said second chamber, pushing at least a portion of the melted metal remaining in the first chamber into said second chamber, and injecting the melted metal from the second chamber into a mold.
The improved system comprises a feeder in which the metal is melted. Melted metal is allowed to flow from the feeder through a feeder port into a first chamber. At least a portion of the melted metal is drawn into a second chamber, assisted by suction through an outlet port leading from the first chamber into the second chamber. A ram in the first chamber pushes some of the remaining melted metal from the first chamber through the outlet port leading into the second chamber, thereby forcing out gas that has accumulated in the second chamber between the melted metal and a piston (commonly referred to as the xe2x80x9cplungerxe2x80x9d) that is positioned inside the second chamber. The pressure from the melted metal being driven into the second chamber by the ram forces the gas between the melted metal and the piston to flow past the piston through the small space between the piston and the wall of the second chamber. The piston in the second chamber then injects the melted metal, which is substantially gas-free, into a mold. Before the injection, the piston in the second chamber is retracted to draw in the melted metal from the first chamber by creating suction and also to regulate the volume of melted metal that is held in the second chamber prior to injection so that it precisely corresponds to the size of the molded part.
The above-described process and system provide a very precise control of the injection volume, to within xc2x10.5% by weight or less, because the injection volume is determined in accordance with the position of the piston and any gas that is present in the melted metal, which can be about 20% by volume, is forced out by operation the ram advancing, before the melted metal is injected.
Further, a fine die-cast method according to the invention is more advantageous than current thixotropic processes because conversion of metal into the thixotropic state takes more time. With the fine die-cast method according to the invention, the injection cycle time is reduced to about 30 seconds, a 50% reduction when compared to current thixotropic processes.
Also, the method of the present invention can be used to mold parts of a liquid material that are more preferred than parts molded from current thixotropic processes. They generally require less post-molding processing, given their more accurate molding volume and smooth surfaces. This permits a production process that is stable over many runs.
In addition, the method of the present invention can provide molded parts of extremely fine dimensions, having thicknesses less than 1 mm for a rectangular-shaped part measuring about 21.0 cm by 29.7 cm (which is roughly the size of a DIN size A4 sheet of paper) and also having more complex structures.
Additional objects and advantages of the invention will be set forth in the description which follows. The objects and advantages of the invention may be realized and obtained by means of instrumentalities and combinations particularly pointed out in the appended claims.